1. Field of the Invention
The invention relates to a valve arrangement for a hydraulic transmission circuit with a clutch valve arrangement for controlling at least one clutch and/or start-up clutch.
2. Discussion of the Background
A valve arrangement is known from U.S. Pat. No. 4,293,059. Here a pressure-regulating valve designated as an "inching valve" is used to control various transmission brakes and clutches. The governing spring is braced so that the characteristic line depends directly on the clutch pedal position. When the pedal is fully depressed, the "inching valve" connects the clutches to the reservoir and the transmission brake to system pressure, but this valve operation occurs in a way purely dependent on spring force, i.e., by changing the force of the governing spring
A similar valve arrangement is shown in U.S. Pat. No. 4,093,051. Here a valve plunger is provided that can be pressed against a clutch pedal under the effect of a spring braced on the regulating valve piston. When the clutch pedal is released, this plunger presses the regulating piston to full output. When the pedal is depressed, the regulating piston reacts to the spring and pressure forces acting on it.
These known valve arrangements no longer sufficiently meet safety requirements, which have meanwhile become more strict, for transmission control valves. Especially when such a valve arrangement is used to control start-up clutches, it is guaranteed that there are maximum safety margins when the vehicle is started and driven off. Here the following requirements are to be fulfilled:
When the vehicle is started, the start-up clutch, which is generally constructed as a multiple-disk clutch that can be closed by pressure, is to be controlled by regulated pressure. Simultaneously, when the clutch pedal is fully depressed, it is certain that the start-up clutch is evacuated automatically and as quickly as possible When the clutch pedal is released, finally the intake of the pressure-regulating valve is opened so wide that slipping of the clutches, even taking into account differing leakage rates in the area of the individual clutches, is precluded. Finally, the arrangement is to be made so that, with slight structural changes, it is possible to provide additional safety functions, such as, e.g., protection against an unintentional, sudden driving off of the vehicle (i.e., a so-called "jump start"). Such a sudden, unexpected jump of the vehicle, such as, e.g., in an agricultural machine, occurs with conventional valve arrangements when, for example, the machine, having engine-starting problems, is started by short-circuiting the starter. In this case, the operator generally stands right in front of the large rear wheel of the vehicle. With mechanical transmissions, the danger to the operator during such an action is less than with change-under-load transmissions. If, namely, with mechanical transmissions, a gear is erroneously engaged due to inattention and then short-circuiting of the starter occurs, then the vehicle already begins to jerk with the first rotation of the starter and the starting process can be broken off. If, on the other hand, in change-under-load transmissions, a gear remains engaged, this state cannot be recognized right away. Rather, it generally takes a certain amount of time until the starting engine and the pump it drives have built up enough pressure to close the clutches. Then in any case the vehicle begins to move suddenly with a jump, greatly endangering the operator.